US20230193417A1 - Hot Stamped Part and Method for Manufacturing the Same - Google Patents
Hot Stamped Part and Method for Manufacturing the Same Download PDFInfo
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- US20230193417A1 US20230193417A1 US18/169,277 US202318169277A US2023193417A1 US 20230193417 A1 US20230193417 A1 US 20230193417A1 US 202318169277 A US202318169277 A US 202318169277A US 2023193417 A1 US2023193417 A1 US 2023193417A1
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- base metal
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
- C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21D22/00—Shaping without cutting, by stamping, spinning, or deep-drawing
- B21D22/02—Stamping using rigid devices or tools
- B21D22/022—Stamping using rigid devices or tools by heating the blank or stamping associated with heat treatment
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21D37/00—Tools as parts of machines covered by this subclass
- B21D37/16—Heating or cooling
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B32—LAYERED PRODUCTS
- B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
- B32B15/00—Layered products comprising a layer of metal
- B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
- B32B15/012—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of aluminium or an aluminium alloy
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B32—LAYERED PRODUCTS
- B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
- B32B15/00—Layered products comprising a layer of metal
- B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
- B32B15/013—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/62—Quenching devices
- C21D1/673—Quenching devices for die quenching
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/002—Heat treatment of ferrous alloys containing Cr
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/005—Heat treatment of ferrous alloys containing Mn
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/008—Heat treatment of ferrous alloys containing Si
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/002—Bainite
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/008—Martensite
Definitions
- the present disclosure relates to a hot stamped part and a method for manufacturing the same, and more particularly to a hot stamped part, which has improved toughness while maintaining high strength and high hardness, and a method for manufacturing the same.
- hot stamping is a method in which a plate is heated to a high temperature of 900° C. or higher and is then forming and cooling are simultaneously performed using a press in which cooling water flows so as to form a product and, because the product acquired by hot stamping may have a complicated shape and secure excellent dimensional precision and high strength, hot stamping is being applied as a method for manufacturing various parts for vehicles.
- a part manufactured by hot stamping is transformed to have the full austenite structure when it is heat-treated at the A3 temperature or higher, and finally forms the martensite structure due to cooling during hot stamping. Therefore, the part manufactured by hot stamping has very high strength and high hardness, but has insufficient toughness and is thus cracked in a collision test.
- the strength of the lower end of the center pillar reinforcement in the length direction of a center pillar is decreased by local softening, or a steel plate having low strength is applied to the lower end of the center pillar reinforcement using the tailor welded blank (TWB) technology.
- TWB tailor welded blank
- the present disclosure has been made in view of the above problems, and it is an object of the present disclosure to provide a hot stamped part, which has improved toughness while maintaining high strength and high hardness so as to secure excellent bendability by controlling formation of microstructures in the thickness direction thereof, and a method for manufacturing the same.
- a hot stamped part formed by performing hot stamping using an iron-based alloy, the hot stamped part including a reinforced portion formed to have a martensite structure, a softened portion formed to have ferrite and bainite structures, and a transition portion formed between the reinforced portion and the softened portion, wherein the reinforced portion, the transition portion and the softened portion are formed in a thickness direction of the hot stamped part.
- a ratio of a thickness of the softened portion to a total thickness of the hot stamped part may be equal to or less than 30%.
- the transition potion may be formed to have the ferrite, bainite and martensite structures.
- the iron-based alloy may include 0.19-0.25 wt % of C, 0.40 wt % or less of Si, 1.10-1.60 wt % of Mn, 0.030 wt % or less of P, 0.015 wt % or less of S, 0.10-0.60 wt % of Cr, 0.0008-0.0050 wt % of B, the balance of Fe, and inevitable impurities.
- a tensile strength of the hot stamped part may be equal to or greater than 1300 MPa.
- a bending angle of the hot stamped part may be equal to or greater than 90°.
- a plating layer formed of an Al—Si-based alloy or a Zn—based alloy may be further formed on each of surfaces of the reinforced portion and the softened portion.
- a method for manufacturing a hot stamped part including preparing a plate-type base metal using an iron-based alloy, heating the prepared base metal, forming a product by inserting the heated base metal between a first die and a second die and then pressing the base metal, and cooling the product formed between the first die and the second die while differently maintaining a cooling speed of one surface of the product configured to come into contact with the first die and a cooling speed of a remaining surface of the product configured to come into contact with the second die.
- the first die may be a heated die and the second die is a cooled die, and the heated base metal may be placed on the first die.
- a cavity having a recessed shape may be formed in the first die, a protrusion having a projecting shape may be formed on the second die so as to be inserted into the cavity, and a heated heating pad may be disposed in the cavity of the first die, and, in the forming the product, when the heated base metal is placed on the first die, the heated base metal may be placed on an upper surface of the first die and an upper surface of the heating pad.
- the heating pad disposed in the cavity of the first die may be inserted into the first die by pressing force of the second die, and thus, a space configured such that the heated base metal is formed into the product may be secured.
- a softened portion configured to have ferrite and bainite structures may be formed from the surface of the product configured to come into contact with the first die, and a reinforced portion configured to have a martensite structure may be formed from the remaining surface of the product configured to come into contact with the second die.
- the softened portion may be formed such that a ratio of a thickness of the softened portion to a total thickness of the product is equal to or less than 30%.
- the cooling speed of the surface of the product configured to come into contact with the first die may be 3-5 ° C./s in a temperature section of 850-500° C.
- the cooling speed of the remaining surface of the product configured to come into contact with the second die may be equal to or higher than 27 ° C./s in a temperature section of 850-250° C.
- the first die may be heated to a temperature range of 300-450° C.
- FIG. 1 is a cross-sectional view of a hot stamped part according to one embodiment of the present disclosure, in the thickness direction thereof;
- FIG. 2 A illustrates a first step in a method for manufacturing a hot stamped part according to one embodiment of the present disclosure
- FIG. 2 B illustrates a second step in the method of FIG. 2 A for manufacturing a hot stamped part according to one embodiment of the present disclosure
- FIG. 2 C illustrates a hot stamped part resulting from the methods of FIGS. 2 A- 2 B according to one embodiment of the present disclosure
- FIG. 3 is a graph showing results of a bending test of an example and a comparative example of the present disclosure.
- FIG. 1 is a cross-sectional view of a hot stamped part according to one embodiment of the present disclosure, in the thickness direction thereof.
- the hot stamped part is a part formed by performing hot stamping using an iron-based alloy, and 22MnB5, which is generally used as an alloy for hot stamping, is applied as the iron-based alloy.
- the iron-based alloy includes 0.19-0.25 wt % of C, 0.40 wt % or less of Si, 1.10-1.60 wt % of Mn, 0.030 wt % or less of P, 0.015 wt % or less of S, 0.10-0.60 wt % of Cr, 0.0008-0.0050 wt % of B, the balance of Fe, and inevitable impurities.
- the hot stamped part according to one embodiment of the present disclosure using the iron-based alloy, which is generally used as an alloy for hot stamping, is a part which has improved toughness while maintaining high strength and high hardness so as to secure excellent bendability by forming different microstructures in the thickness direction thereof, in contrast to a conventional hot stamped part which is formed to have the full martensite structure and thus has high strength and high hardness but has low toughness.
- a hot stamped part 10 includes a reinforced portion 11 configured to be formed of the martensite structure after hot stamping, a softened portion 12 configured to be formed of the ferrite and bainite structures, and a transition portion 13 formed between the reinforced portion 11 and the softened portion 12 .
- the reinforced portion 11 , the transition portion 13 and the softened portion 12 are sequentially formed in the thickness direction of the hot stamped part 10 .
- the reinforced portion 11 is a region configured to form the hot stamped part 10 , and is formed by transforming a base metal, which is formed of the ferrite and perlite structures at room temperature, to the full austenite structure by heating the base metal to the A3 temperature or higher, and then transforming the full austenite structure to the full martensite structure by rapidly cooling the full austenite structure to room temperature.
- the reinforced portion 11 serves to maintain stiffness required by a collision member when the hot stamped part is applied to the collision member.
- the softened portion 12 is a region configured to improve bendability of the hot stamped part 10 , and is formed by transforming a base metal, which is formed of the ferrite and perlite structures at room temperature, to the full austenite structure by heating the base metal to the A3 temperature or higher, and then transforming the full austenite structure to the ferrite and bainite structures by slowly cooling the full austenite structure to room temperature.
- the softened portion 12 serves to improve bendability required by a collision member when the hot stamped part is applied to the collision member.
- the reinforced portion 11 is formed from one surface of the hot stamped part 10
- the softened portion 12 is formed from the other surface of the hot stamped part 10
- the transition portion 13 in which all of the ferrite, bainite and martensite structures are formed, is formed between the reinforced portion 11 and the softened portion 12 .
- the transition portion 13 serves as a buffer between the reinforced portion 11 configured to maintain stiffness of the hot stamped part 10 and the softened portion 12 configured to improve bendability of the hot stamped part 10 .
- the softened portion 12 may be formed to have a thickness which is equal to or less than 30% of the total thickness of the hot stamped part 10 .
- the thickness of the softened portion 12 exceeds 30% of the total thickness of the hot stamped part 10 , the toughness of the hot stamped part 10 is increased and thus the bendability thereof is improved, whereas the tensile strength of the hot stamped part 10 is greatly reduced and thus the hot stamped part 10 does not serve as a collision member.
- the hot stamped part 10 may have a tensile strength of 1300 MPa or more and a bending angle of 90° or more.
- the hot stamped part 10 may further include a plating layer 14 which is formed of an Al—Si-based alloy or a Zn-based alloy so as to cover each of the surfaces of the reinforced portion 11 and the softened portion 12 .
- the plating layers 14 are formed on the outermost surfaces of the hot stamped part 10 , thereby improving corrosion resistance of the surfaces of the hot stamped part 10 .
- the plating layers 14 formed on the surfaces of the reinforced portion 11 and the softened portion 12 are not limited to an Al—Si-based alloy or a Zn-based alloy, and plating layers which are formed of various materials so as to protect the surfaces of the hot stamped part 10 may be formed depending on specifications applied to the hot stamped part 10 .
- FIGS. 2 A to 2 C are views illustrating the method for manufacturing the hot stamped part according to one embodiment of the present disclosure.
- the die set for hot stamping is an apparatus which produces a hot stamped part by inserting a heated base metal 10 a between a first die 100 and a second die 200 and then pressing the heated base metal 10 a so as to form a product, the first die 100 is a heated die, and the second die 200 is a cooled die.
- the heated die means a die which is directly or indirectly heated
- the cooled die means a die which is directly or indirectly cooled.
- the first die 100 may be heated to the temperature range of a desired level by allowing a heated fluid to flow thereinto
- the second die 200 may be cooled to the temperature range of a desired level by allowing a cooled fluid to flow thereinto.
- the heated base metal 10 a may be placed on the first die 100 .
- the first die 100 may be disposed as a lower die
- the second die 200 may be disposed as an upper die.
- a cavity 100 a having a recessed shape is formed in the first die 100
- a protrusion 200 a having a projecting shape, which is inserted into the cavity 100 a is formed on the second die 200 .
- a heated heating pad 300 may further disposed in the cavity 100 a of the first die 100 .
- the heated heating pad 300 is disposed in the cavity 100 a so that the surface of the heated base metal 10 a comes into contact with the upper surface of the first die 100 and the upper surface of the heating pad 300 . Thereby, rapid cooling of one surface of the heated base metal 10 a may be suppressed.
- a recess 110 into which the heating pad 300 is inserted, is formed in the first die 100 so as to secure a space in which the base metal 10 a is formed into a product while removing the heating pad 300 from the cavity 100 a of the first die 100 during pressing.
- the recess 110 having a shape corresponding to the shape of the heating pad 300 is formed under the cavity 100 a . Therefore, while the second die 200 is pressed from above the first die 100 , the heating pad 300 is inserted into the recess 110 by the pressing force of the second die 200 .
- the upper surface of the heating pad 300 is used as the lower surface of the cavity 100 a formed in the first die 100 .
- One or more guide grooves 120 which guide movement of the heating pad 300 , are formed under the recess 110 , and guide bars 310 , which are inserted into the guide grooves 120 so as to be guided, are formed on the lower surface of the heating pad 300 . Thereby, movement of the heating pad 300 is guided when the heating pad 300 is moved upwards and downwards.
- elastic members 320 which provide restoring force to withdraw the heating pad 300 towards the cavity 100 a , are installed in the guide grooves 120 .
- coil springs may be used as the elastic members 320 .
- the heating pad 300 is moved downwards and thus inserted into the recess 110 and, when the pressing force of the second die 200 is released, the heating pad 300 is withdrawn towards the cavity 100 a by the restoring force of the elastic members 320 . Further, while the second die 200 is pressed onto the first die 100 , the heating pad 300 is moved downwards in the state in which the heating pad 300 is pressed against one surface of the heated base metal 10 a by the elastic members 320 , thereby being capable of delaying cooling of the heated base metal 10 a.
- a method for manufacturing a hot stamped part includes preparing a plate-type base metal 10 a using an iron-based alloy, heating the prepared base metal 10 a , forming a product 10 b by inserting the heated base metal 10 a between the first die 100 and the second die 200 and then pressing the base metal 10 a , and cooling the product 10 b formed between the first die 100 and the second die 200 while differently maintaining a cooling speed of one surface of the product 10 b which comes into contact with the first die 100 and a cooling speed of the other surface of the product 10 b which comes into contact with the second die 200 .
- microstructures of the base metal 10 a include the ferrite and bainite structures.
- the microstructures of the base metal 10 a are transformed into the full austenite structure by heating the prepared base metal 10 a to the A3 temperature or higher.
- the base metal 10 a is heated to a temperature of 900° C. or higher.
- the product 10 b is formed by inserting the heated base metal 10 a between the first die 100 and the second die 200 and then pressing the base metal 10 a .
- the heated base metal 10 a is placed on the upper surface of the heated first die 100 .
- one surface of the heated base metal 10 a is placed on the upper surface of the first die 100 and the upper surface of the heating pad 300 .
- the second die 200 is moved downwards so as to press the heated base metal 10 a . Then, while the second die 200 comes close to the first die 100 and thus presses the heated base metal 10 a placed on the first die 100 , the heating pad 300 disposed in the cavity 100 a of the first die 100 is inserted into the recess 110 of the first die 100 by the pressing force of the second die 200 , and thus, the space in which the heated base metal 10 a is formed into the product 10 b is secured.
- the state, in which the heating pad 300 is pressed against one surface of the heated base metal 10 a is maintained by the elastic members 320 .
- the product 10 b is cooled.
- the product 10 b formed between the first die 100 and the second die 200 is cooled while differently maintaining the cooling speed of one surface of the product 10 b which comes into contact with the first die 100 and the cooling speed of the other surface of the product 10 b which comes into contact with the second die 200 .
- one surface of the product 10 b is slowly cooled by the first die 100 , and the other surface of the product 10 b is rapidly cooled by the second die 200 .
- one surface of the product 10 b is slowly cooled to room temperature and thus transformed into the ferrite and bainite structures, thereby forming a softened portion 12 .
- the other surface of the product 10 b is rapidly cooled to room temperature and thus transformed into the full martensite structure, thereby forming a reinforced portion 11 .
- the transition portion 13 in which all of the ferrite, bainite and martensite structures are formed, is formed between the reinforced portion 11 and the softened portion 12 .
- the cooling speed of the product 10 b due to the first die 100 is adjusted so that the thickness of the softened portion 12 has a ratio of 30% or less to the total thickness of the product 10 b.
- the cooling speed of the surface of the product 10 b which comes into contact with the first die 100 , is 3-5° C./s in a temperature section of 850-500° C.
- the cooling speed of the other surface of the product 10 b which comes into contact with the second die 200 , is equal to or higher than 27° C./s in a temperature section of 850-250° C.
- the first die 100 is heated to a temperature range of 300-450° C.
- a plate formed of 22MnB5 as an alloy was heated to a temperature of 950° C. so as to have the full austenite structure, and then, forming and cooling of the plate were performed using the hot stamping apparatus according to the present disclosure.
- the heating temperature of the first die was changed, as set forth in Table 1 below, and the ratios of softened portions formed thereby were measured, as set forth in Table 1 below.
- the ratio of the thickness of the softened portion to the total thickness of the product is 30% or less.
- a 3-point bending test i.e., the VDA238-100 test
- a conventional general hot stamped part i.e., a hot stamped part having the full martensite structure
- a hot stamped part according to the present disclosure i.e., a hot stamped part having a ratio of the thickness of a softened portion thereof, which is 21%, is formed, and results of the test are shown in FIG. 3 .
- the hot stamped part according to the example of the present disclosure maintains a bending angle of 90° or more.
- the martensite structure may be formed on one surface of a product in the thickness direction thereof and the ferrite and bainite structures may be formed on the other surface of the product by differently controlling the cooling speeds of both surfaces of the product during hot stamping.
- the hot stamped part which has improved toughness while maintaining high strength and high hardness so as to secure excellent bendability, may be manufactured.
- a collision member having excellent performance such as a center pillar for vehicles, may be manufactured by a comparatively simple process.
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- Materials Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
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- Crystallography & Structural Chemistry (AREA)
- Shaping Metal By Deep-Drawing, Or The Like (AREA)
- Heat Treatment Of Articles (AREA)
Abstract
Description
- This application is a divisional application of U.S. Ser. No. 17/307,100, filed May 4, 2021, which claims priority from Korean Patent Application No. 10-2020-0151949, filed on Nov. 13, 2020, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety.
- The present disclosure relates to a hot stamped part and a method for manufacturing the same, and more particularly to a hot stamped part, which has improved toughness while maintaining high strength and high hardness, and a method for manufacturing the same.
- In general, hot stamping is a method in which a plate is heated to a high temperature of 900° C. or higher and is then forming and cooling are simultaneously performed using a press in which cooling water flows so as to form a product and, because the product acquired by hot stamping may have a complicated shape and secure excellent dimensional precision and high strength, hot stamping is being applied as a method for manufacturing various parts for vehicles.
- A part manufactured by hot stamping is transformed to have the full austenite structure when it is heat-treated at the A3 temperature or higher, and finally forms the martensite structure due to cooling during hot stamping. Therefore, the part manufactured by hot stamping has very high strength and high hardness, but has insufficient toughness and is thus cracked in a collision test.
- Among parts applied to vehicles, in case of a center pillar reinforcement manufactured by hot stamping, when the center pillar reinforcement is cracked, it does not serve as a collision member any more, and thus does not protect passengers due to the increased intrusive volume thereof.
- In order to compensate for the defect of the center pillar reinforcement manufactured by hot stamping, the strength of the lower end of the center pillar reinforcement in the length direction of a center pillar is decreased by local softening, or a steel plate having low strength is applied to the lower end of the center pillar reinforcement using the tailor welded blank (TWB) technology.
- The above information disclosed in the Background section is only for enhancement of understanding of the background of the disclosure and should not be interpreted as conventional technology that is already known to those skilled in the art.
- Therefore, the present disclosure has been made in view of the above problems, and it is an object of the present disclosure to provide a hot stamped part, which has improved toughness while maintaining high strength and high hardness so as to secure excellent bendability by controlling formation of microstructures in the thickness direction thereof, and a method for manufacturing the same.
- In accordance with an aspect of the present disclosure, the above and other objects can be accomplished by the provision of a hot stamped part, formed by performing hot stamping using an iron-based alloy, the hot stamped part including a reinforced portion formed to have a martensite structure, a softened portion formed to have ferrite and bainite structures, and a transition portion formed between the reinforced portion and the softened portion, wherein the reinforced portion, the transition portion and the softened portion are formed in a thickness direction of the hot stamped part.
- A ratio of a thickness of the softened portion to a total thickness of the hot stamped part may be equal to or less than 30%.
- The transition potion may be formed to have the ferrite, bainite and martensite structures.
- The iron-based alloy may include 0.19-0.25 wt % of C, 0.40 wt % or less of Si, 1.10-1.60 wt % of Mn, 0.030 wt % or less of P, 0.015 wt % or less of S, 0.10-0.60 wt % of Cr, 0.0008-0.0050 wt % of B, the balance of Fe, and inevitable impurities.
- A tensile strength of the hot stamped part may be equal to or greater than 1300 MPa.
- A bending angle of the hot stamped part may be equal to or greater than 90°.
- A plating layer formed of an Al—Si-based alloy or a Zn—based alloy may be further formed on each of surfaces of the reinforced portion and the softened portion.
- In accordance with another aspect of the present disclosure, there is provided a method for manufacturing a hot stamped part, the method including preparing a plate-type base metal using an iron-based alloy, heating the prepared base metal, forming a product by inserting the heated base metal between a first die and a second die and then pressing the base metal, and cooling the product formed between the first die and the second die while differently maintaining a cooling speed of one surface of the product configured to come into contact with the first die and a cooling speed of a remaining surface of the product configured to come into contact with the second die.
- In the forming the product, the first die may be a heated die and the second die is a cooled die, and the heated base metal may be placed on the first die.
- In the forming the product, a cavity having a recessed shape may be formed in the first die, a protrusion having a projecting shape may be formed on the second die so as to be inserted into the cavity, and a heated heating pad may be disposed in the cavity of the first die, and, in the forming the product, when the heated base metal is placed on the first die, the heated base metal may be placed on an upper surface of the first die and an upper surface of the heating pad.
- In the forming the product, while the second die comes close to the first die and thus presses the heated base metal placed on the first die, the heating pad disposed in the cavity of the first die may be inserted into the first die by pressing force of the second die, and thus, a space configured such that the heated base metal is formed into the product may be secured.
- In the cooling the product, a softened portion configured to have ferrite and bainite structures may be formed from the surface of the product configured to come into contact with the first die, and a reinforced portion configured to have a martensite structure may be formed from the remaining surface of the product configured to come into contact with the second die.
- In the cooling the product, the softened portion may be formed such that a ratio of a thickness of the softened portion to a total thickness of the product is equal to or less than 30%.
- In the cooling the product, the cooling speed of the surface of the product configured to come into contact with the first die may be 3-5 ° C./s in a temperature section of 850-500° C., and the cooling speed of the remaining surface of the product configured to come into contact with the second die may be equal to or higher than 27 ° C./s in a temperature section of 850-250° C.
- In the forming the product, the first die may be heated to a temperature range of 300-450° C.
- The above and other objects, features and other advantages of the present disclosure will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:
-
FIG. 1 is a cross-sectional view of a hot stamped part according to one embodiment of the present disclosure, in the thickness direction thereof; -
FIG. 2A illustrates a first step in a method for manufacturing a hot stamped part according to one embodiment of the present disclosure; -
FIG. 2B illustrates a second step in the method ofFIG. 2A for manufacturing a hot stamped part according to one embodiment of the present disclosure; -
FIG. 2C illustrates a hot stamped part resulting from the methods ofFIGS. 2A-2B according to one embodiment of the present disclosure; and -
FIG. 3 is a graph showing results of a bending test of an example and a comparative example of the present disclosure. - Hereinafter reference will now be made in detail to various embodiments of the present disclosure, examples of which are illustrated in the accompanying drawings and described below. While the disclosure will be described in conjunction with exemplary embodiments, it will be understood that present description is not intended to limit the disclosure to the exemplary embodiments. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.
-
FIG. 1 is a cross-sectional view of a hot stamped part according to one embodiment of the present disclosure, in the thickness direction thereof. - As shown in
FIG. 1 , the hot stamped part according to one embodiment of the present disclosure is a part formed by performing hot stamping using an iron-based alloy, and 22MnB5, which is generally used as an alloy for hot stamping, is applied as the iron-based alloy. For example, the iron-based alloy includes 0.19-0.25 wt % of C, 0.40 wt % or less of Si, 1.10-1.60 wt % of Mn, 0.030 wt % or less of P, 0.015 wt % or less of S, 0.10-0.60 wt % of Cr, 0.0008-0.0050 wt % of B, the balance of Fe, and inevitable impurities. - The hot stamped part according to one embodiment of the present disclosure using the iron-based alloy, which is generally used as an alloy for hot stamping, is a part which has improved toughness while maintaining high strength and high hardness so as to secure excellent bendability by forming different microstructures in the thickness direction thereof, in contrast to a conventional hot stamped part which is formed to have the full martensite structure and thus has high strength and high hardness but has low toughness.
- For example, a hot stamped
part 10 according to one embodiment of the present disclosure includes a reinforcedportion 11 configured to be formed of the martensite structure after hot stamping, a softenedportion 12 configured to be formed of the ferrite and bainite structures, and atransition portion 13 formed between the reinforcedportion 11 and the softenedportion 12. Here, the reinforcedportion 11, thetransition portion 13 and the softenedportion 12 are sequentially formed in the thickness direction of the hot stampedpart 10. - The reinforced
portion 11 is a region configured to form the hot stampedpart 10, and is formed by transforming a base metal, which is formed of the ferrite and perlite structures at room temperature, to the full austenite structure by heating the base metal to the A3 temperature or higher, and then transforming the full austenite structure to the full martensite structure by rapidly cooling the full austenite structure to room temperature. Thereby, the reinforcedportion 11 serves to maintain stiffness required by a collision member when the hot stamped part is applied to the collision member. - The softened
portion 12 is a region configured to improve bendability of the hot stampedpart 10, and is formed by transforming a base metal, which is formed of the ferrite and perlite structures at room temperature, to the full austenite structure by heating the base metal to the A3 temperature or higher, and then transforming the full austenite structure to the ferrite and bainite structures by slowly cooling the full austenite structure to room temperature. Thereby, the softenedportion 12 serves to improve bendability required by a collision member when the hot stamped part is applied to the collision member. - During the process for cooling the hot stamped
part 10, the reinforcedportion 11 is formed from one surface of the hot stampedpart 10, the softenedportion 12 is formed from the other surface of the hot stampedpart 10, and thereby, thetransition portion 13, in which all of the ferrite, bainite and martensite structures are formed, is formed between the reinforcedportion 11 and the softenedportion 12. - The
transition portion 13 serves as a buffer between the reinforcedportion 11 configured to maintain stiffness of the hot stampedpart 10 and the softenedportion 12 configured to improve bendability of the hot stampedpart 10. - Particularly, the softened
portion 12 may be formed to have a thickness which is equal to or less than 30% of the total thickness of the hot stampedpart 10. When the thickness of the softenedportion 12 exceeds 30% of the total thickness of the hot stampedpart 10, the toughness of the hot stampedpart 10 is increased and thus the bendability thereof is improved, whereas the tensile strength of the hot stampedpart 10 is greatly reduced and thus the hot stampedpart 10 does not serve as a collision member. - Further, in order to apply the hot stamped
part 10 according to one embodiment of the present disclosure to a collision member, such as a center pillar which is a part for vehicles, the hot stampedpart 10 may have a tensile strength of 1300 MPa or more and a bending angle of 90° or more. - The hot stamped
part 10 according to one embodiment of the present disclosure may further include aplating layer 14 which is formed of an Al—Si-based alloy or a Zn-based alloy so as to cover each of the surfaces of the reinforcedportion 11 and the softenedportion 12. As such, the platinglayers 14 are formed on the outermost surfaces of the hot stampedpart 10, thereby improving corrosion resistance of the surfaces of the hot stampedpart 10. Of course, theplating layers 14 formed on the surfaces of the reinforcedportion 11 and the softenedportion 12 are not limited to an Al—Si-based alloy or a Zn-based alloy, and plating layers which are formed of various materials so as to protect the surfaces of the hot stampedpart 10 may be formed depending on specifications applied to the hot stampedpart 10. - Next, a method for manufacturing a hot stamped part according to one embodiment of the present disclosure and a die set for hot stamping used to manufacture the hot stamped part will be described.
-
FIGS. 2A to 2C are views illustrating the method for manufacturing the hot stamped part according to one embodiment of the present disclosure. - First, the die set to which the method for manufacturing the hot stamped part according to one embodiment of the present disclosure is applied will be described.
- As shown in
FIGS. 2A to 2C , the die set for hot stamping is an apparatus which produces a hot stamped part by inserting aheated base metal 10 a between afirst die 100 and asecond die 200 and then pressing theheated base metal 10 a so as to form a product, thefirst die 100 is a heated die, and thesecond die 200 is a cooled die. Here, the heated die means a die which is directly or indirectly heated, and the cooled die means a die which is directly or indirectly cooled. For example, thefirst die 100 may be heated to the temperature range of a desired level by allowing a heated fluid to flow thereinto, and thesecond die 200 may be cooled to the temperature range of a desired level by allowing a cooled fluid to flow thereinto. - Further, in this embodiment, in order to prevent the
heated base metal 10 a from being rapidly cooled when theheated base metal 10 a is placed between thefirst die 100 and thesecond die 200, theheated base metal 10 a may be placed on thefirst die 100. For this purpose, in this embodiment, thefirst die 100 may be disposed as a lower die, and thesecond die 200 may be disposed as an upper die. - Further, a
cavity 100 a having a recessed shape is formed in thefirst die 100, and aprotrusion 200 a having a projecting shape, which is inserted into thecavity 100 a, is formed on thesecond die 200. - Particularly, in this embodiment, a
heated heating pad 300 may further disposed in thecavity 100 a of thefirst die 100. In order to prevent one surface of theheated base metal 10 a from being exposed to thecavity 100 a and being thus cooled when the surface of theheated base metal 10 a is placed on the upper surface of thefirst die 100, theheated heating pad 300 is disposed in thecavity 100 a so that the surface of theheated base metal 10 a comes into contact with the upper surface of thefirst die 100 and the upper surface of theheating pad 300. Thereby, rapid cooling of one surface of theheated base metal 10 a may be suppressed. - A
recess 110, into which theheating pad 300 is inserted, is formed in thefirst die 100 so as to secure a space in which thebase metal 10 a is formed into a product while removing theheating pad 300 from thecavity 100 a of thefirst die 100 during pressing. Here, therecess 110 having a shape corresponding to the shape of theheating pad 300 is formed under thecavity 100 a. Therefore, while thesecond die 200 is pressed from above thefirst die 100, theheating pad 300 is inserted into therecess 110 by the pressing force of thesecond die 200. In this case, the upper surface of theheating pad 300 is used as the lower surface of thecavity 100 a formed in thefirst die 100. - One or
more guide grooves 120, which guide movement of theheating pad 300, are formed under therecess 110, and guidebars 310, which are inserted into theguide grooves 120 so as to be guided, are formed on the lower surface of theheating pad 300. Thereby, movement of theheating pad 300 is guided when theheating pad 300 is moved upwards and downwards. Further,elastic members 320, which provide restoring force to withdraw theheating pad 300 towards thecavity 100 a, are installed in theguide grooves 120. For example, coil springs may be used as theelastic members 320. Therefore, while thesecond die 200 is pressed onto thefirst die 100, theheating pad 300 is moved downwards and thus inserted into therecess 110 and, when the pressing force of thesecond die 200 is released, theheating pad 300 is withdrawn towards thecavity 100 a by the restoring force of theelastic members 320. Further, while thesecond die 200 is pressed onto thefirst die 100, theheating pad 300 is moved downwards in the state in which theheating pad 300 is pressed against one surface of theheated base metal 10 a by theelastic members 320, thereby being capable of delaying cooling of theheated base metal 10 a. - Next, a method for manufacturing a hot stamped part using the above-described hot stamping apparatus will be described.
- A method for manufacturing a hot stamped part according to one embodiment of the present disclosure includes preparing a plate-
type base metal 10 a using an iron-based alloy, heating theprepared base metal 10 a, forming aproduct 10 b by inserting theheated base metal 10 a between thefirst die 100 and thesecond die 200 and then pressing thebase metal 10 a, and cooling theproduct 10 b formed between thefirst die 100 and thesecond die 200 while differently maintaining a cooling speed of one surface of theproduct 10 b which comes into contact with thefirst die 100 and a cooling speed of the other surface of theproduct 10 b which comes into contact with thesecond die 200. - In the preparation of the
base metal 10 a, 22MnB5, which is generally used as an alloy for hot stamping, is applied into a plate type. - Here, microstructures of the
base metal 10 a include the ferrite and bainite structures. - In the heating of the
base metal 10 a, the microstructures of thebase metal 10 a are transformed into the full austenite structure by heating theprepared base metal 10 a to the A3 temperature or higher. For example, in the heating of thebase metal 10 a, thebase metal 10 a is heated to a temperature of 900° C. or higher. - In the formation of the
product 10 b, theproduct 10 b is formed by inserting theheated base metal 10 a between thefirst die 100 and thesecond die 200 and then pressing thebase metal 10 a. First, theheated base metal 10 a is placed on the upper surface of the heatedfirst die 100. Here, one surface of theheated base metal 10 a is placed on the upper surface of thefirst die 100 and the upper surface of theheating pad 300. - In this state, the
second die 200 is moved downwards so as to press theheated base metal 10 a. Then, while thesecond die 200 comes close to thefirst die 100 and thus presses theheated base metal 10 a placed on thefirst die 100, theheating pad 300 disposed in thecavity 100 a of thefirst die 100 is inserted into therecess 110 of thefirst die 100 by the pressing force of thesecond die 200, and thus, the space in which theheated base metal 10 a is formed into theproduct 10 b is secured. Here, the state, in which theheating pad 300 is pressed against one surface of theheated base metal 10 a, is maintained by theelastic members 320. - Subsequent to the formation of the
product 10 b, theproduct 10 b is cooled. - In the cooling of the
product 10 b, theproduct 10 b formed between thefirst die 100 and thesecond die 200 is cooled while differently maintaining the cooling speed of one surface of theproduct 10 b which comes into contact with thefirst die 100 and the cooling speed of the other surface of theproduct 10 b which comes into contact with thesecond die 200. - In more detail, one surface of the
product 10 b is slowly cooled by thefirst die 100, and the other surface of theproduct 10 b is rapidly cooled by thesecond die 200. - Thereby, one surface of the
product 10 b is slowly cooled to room temperature and thus transformed into the ferrite and bainite structures, thereby forming a softenedportion 12. Further, the other surface of theproduct 10 b is rapidly cooled to room temperature and thus transformed into the full martensite structure, thereby forming a reinforcedportion 11. Further, thetransition portion 13, in which all of the ferrite, bainite and martensite structures are formed, is formed between the reinforcedportion 11 and the softenedportion 12. - In the cooling of the
product 10 b, the cooling speed of theproduct 10 b due to thefirst die 100 is adjusted so that the thickness of the softenedportion 12 has a ratio of 30% or less to the total thickness of theproduct 10 b. - For example, the cooling speed of the surface of the
product 10 b, which comes into contact with thefirst die 100, is 3-5° C./s in a temperature section of 850-500° C., and the cooling speed of the other surface of theproduct 10 b, which comes into contact with thesecond die 200, is equal to or higher than 27° C./s in a temperature section of 850-250° C. - For this purpose, in the formation of the
product 10 b, thefirst die 100 is heated to a temperature range of 300-450° C. - Hereinafter, the present disclosure will be described through examples and a comparative example.
- A plate formed of 22MnB5 as an alloy was heated to a temperature of 950° C. so as to have the full austenite structure, and then, forming and cooling of the plate were performed using the hot stamping apparatus according to the present disclosure.
- Here, the heating temperature of the first die was changed, as set forth in Table 1 below, and the ratios of softened portions formed thereby were measured, as set forth in Table 1 below.
-
TABLE 1 Heating temperature of Ratio of thickness of first die (° C.) softened portion (%) 300 0 400 6 425 12 450 21 475 34 500 43 600 68 - As stated in Table 1, it may be confirmed that, when the heating temperature of the first die is maintained within the range of 300-450° C., the ratio of the thickness of the softened portion to the total thickness of the product is 30% or less.
- Next, the tensile strengths of the products acquired by the above test were measured, and results of the measurement of the tensile strengths are set forth in Table 2 below.
-
TABLE 2 Ratio of thickness of softened portion (%) Tensile strength (MPa) 6 1487 12 1440 21 1379 34 1292 43 1212 - As stated in Table 2, it may be confirmed that, when the ratio of the thickness of the softened portion to the total thickness of the product is maintained in the range of 30% or less, the tensile strength of the product is maintained at 1300 MPa or more.
- Thereafter, a 3-point bending test, i.e., the VDA238-100 test, was conducted on the comparative example, in which a conventional general hot stamped part, i.e., a hot stamped part having the full martensite structure, is formed, and the example, in which a hot stamped part according to the present disclosure, i.e., a hot stamped part having a ratio of the thickness of a softened portion thereof, which is 21%, is formed, and results of the test are shown in
FIG. 3 . - As shown in
FIG. 3 , it may be confirmed that the hot stamped part according to the example of the present disclosure maintains a bending angle of 90° or more. - As is apparent from the above description, in a hot stamped part and a method for manufacturing the same according to the present disclosure, the martensite structure may be formed on one surface of a product in the thickness direction thereof and the ferrite and bainite structures may be formed on the other surface of the product by differently controlling the cooling speeds of both surfaces of the product during hot stamping.
- Thereby, the hot stamped part, which has improved toughness while maintaining high strength and high hardness so as to secure excellent bendability, may be manufactured.
- Therefore, a collision member having excellent performance, such as a center pillar for vehicles, may be manufactured by a comparatively simple process.
- Although the preferred embodiments of the present disclosure have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the disclosure as disclosed in the accompanying claims.
Claims (8)
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US18/169,277 US20230193417A1 (en) | 2020-11-13 | 2023-02-15 | Hot Stamped Part and Method for Manufacturing the Same |
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KR1020200151949A KR20220065433A (en) | 2020-11-13 | 2020-11-13 | Hot stamped parts and manufacturing method thereof |
US17/307,100 US20220154305A1 (en) | 2020-11-13 | 2021-05-04 | Hot Stamped Part and Method for Manufacturing the Same |
US18/169,277 US20230193417A1 (en) | 2020-11-13 | 2023-02-15 | Hot Stamped Part and Method for Manufacturing the Same |
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